Showdown

To get this achievement you will need to do it in one playthrough so if you passed the mission you will need to rewind the story. Also, there will be spoilers.

The first step to doing this achievement is in the mission Karma. When you are chasing down DeFalco you must let him escape so he can survive. After that, you have to fail -- or avoid -- the Strikeforce mission Second Chance. Doing so will also prevent you from getting the Good Karma Achievement / Trophy. It's basically one or the other.

Next, in the mission Achille's Veil when Harper gets caught and you must decide between shooting him or Menendez. Shooting Menendez will get Farid killed and he is necessary for the achievement so shoot Harper instead.

And finally in the mission Odysseus you dont need to do anything just play through to the part where Menendez takes Admiral Briggs. You will then see the cutscene where Salazar betrays Briggs and at the same time Farid and DeFalco will kill each other.

If either Farid or DeFalco are missing from the room you will not get the achievement because you will need them to kill each other, so do not let them die in the previous missions.